1. Field of the Invention
The present invention relates to ultra wideband (UWB) radio communication systems, methods and devices used in the system for generating UWB waveforms that include wavelets that are modulated to convey digital data over a wireless radio communication channel using ultra wideband signaling techniques.
2. Description of the Background
There are numerous radio communication techniques to transmit digital data over a wireless channel. These techniques include those used in mobile telephone systems, pagers, remote data collection systems, and wireless networks for computers, among others. Most conventional wireless communication techniques modulate the digital data onto a high-frequency carrier that is then transmitted via an antenna into space.
Ultra wideband (UWB) communications systems transmit carrierless high data rate, low power signals. Since a carrier is not used, the transmitted waveforms themselves contain the information being communicated. Accordingly, conventional UWB systems transmit pulses, the information to be communicated is contained in the pulses themselves, and not on a carrier.
Conventional UWB communication systems send a sequence of identical pulses, the timing of which carries the information being communicated, for example, as described by Fullerton and Cowie (U.S. Pat. No. 5,677,927). This technique is known as pulse position modulation (PPM). In a PPM scheme, the information in a pulse is obtained by determining an arrival time of the pulse at a receiver relative to other pulses. For example, given an exemplary time window, if a pulse is received at the beginning of that time window, the receiver will decode that pulse as a xe2x80x981,xe2x80x99 whereas if the pulse is received at the end of that same time window, the receiver will decode that pulse as a xe2x80x980.xe2x80x99
Several problems arise with this technique, however, as recognized by the present invention. First, it is not as efficient as other techniques, for example, sending non-inverted and inverted pulses where 3 dB less radiated power is required to communicate in the same memory-less Gaussian white noise channel. Second, reflections from objects in the vicinity of the transmitter and receiver can cause a pulse that was supposed to be at the beginning of the time window, to appear in at the end of time window, or even in the time window of a subsequent pulse.
As a result, it would be advantageous if the data stream to be transmitted could be encoded by changing a shape of the UWB pulse rather than a position of the UWB pulse as with conventional systems. For example, if the UWB pulses had two possible shapes, a single time frame could be used encode a single bit of data, rather than the two time frames (i.e., early and late) that would be required by a PPM system. In the present UWB communications system, and related co-pending application Ser. No. 09/209,460 filed May 14, 1998, entitled ULTRA WIDE BANDWIDTH SPREAD SPECTRUM COMMUNICATIONS SYSTEM, information is carried by the shape of the pulse, or the shape in combination with its position in the pulse-sequence.
Conventional techniques for generating pulses include a variety of techniques, for example, networks of transmission lines such as those described in co-pending application Ser. No. 09/209,460 filed May 14, 1998, entitled ULTRA WIDE BANDWIDTH SPREAD SPECTRUM COMMUNICATIONS SYSTEM. One of the problems associated with this technique is that the transmission lines take up sizeable space and accordingly, are not amenable to integration on a monolithic integrated circuit. Given that a key targeted use of UWB systems is for small, handheld mobile devices such as personal digital assistants (PDAs) and mobile telephones, space is at a premium when designing UWB systems. Furthermore, it is highly desirable to integrate the entire radio onto a single monolithic integrated circuit in order to meet the cost, performance, and volume-production requirements of consumer electronics devices.
A key attribute that must be maintained, however, regardless of how the information is carried, is that no tones can be present. In other words, the average power spectrum must be smooth and void of any spikes. In generating these UWB pulse streams, however, non-ideal device performance can cause tones to pass through to the antenna and to be radiated. In particular, switches, gates, and analog mixers that are used to generate pulses are well known to be non-ideal devices. For example, leakage is a problem. A signal that is supposed to be blocked at certain times, for example, can continue to leak through. Similarly, non-ideal symmetry in positive and negative voltages or current directions can allow tones be generated or leak through. In another example, the output of a mixer can include not only the desired UWB pulse stream, but also spikes in the frequency domain at the clock frequency and its harmonics, as well as other noise, due to leakage between the RF, LO, and IF ports. This is problematic since one of the design objectives is to generate a pulse stream that will not interfere with other communications systems.
Similar problems to those discussed above regarding transmitters are also encountered in UWB receivers. Mixers are used in UWB receivers to mix the received signal with known waveforms so that the transmitted data may be decoded. As discussed above, the spectral spikes (DC and otherwise) introduced by the non-ideal analog mixers can make decoding of only moderately weak signals difficult or impossible.
Furthermore, UWB receivers often suffer from leakage of the UWB signal driving the mixer due to the large amplitude of the drive signal and its very close proximity to the antenna as well as adjacent components. These UWB drive signals can radiate into space and be received by the antenna where it can jam the desired UWB signal, or be coupled via the substrate. This reception of the drive signal being used to decode the received signal can therefore cause a self-jamming condition wherein the desired signal becomes unintelligible.
The challenge, then, as presently recognized, is to develop a highly integratable approach for generating shape-modulated wavelet sequences that can be used in a UWB communications system to encode, broadcast, receive, and decode a data stream. It would be advantageous if the data stream to be transmitted could be encoded by changing a shape of the UWB pulse rather than a position of the UWB pulse as with conventional systems.
Furthermore, the challenge is to build such a wavelet generator where the smooth power spectrum calculated by using ideal components, is realized using non-ideal components. In other words, an approach to generating and receiving UWB waveforms that does not generate unwanted frequency domain spikes as a by-product, spikes that are prone to interfere with other communications devices or cause self-jamming, would be advantageous.
It would also be advantageous if the UWB waveform generation approach were to minimize the power consumption because many of the targeted applications for UWB communications are in handheld battery-operated mobile devices.
Accordingly, one object of this invention is to provide a novel programmable wavelet generator for generating a variety of wavelets for use in a UWB communication system that addresses the above-identified and other problems with conventional devices.
The inventors of the present invention have recognized that by implementing a two-mixer approach to generating UWB waveforms, that the noise leakage from the non-ideal analog mixers can be whitened, thereby avoiding the interference problems caused by conventional single-mixer approaches. The present inventors have provided a contrarian approach of suppressing mixer-created interference by using a second mixer.
The inventors of the present invention have also recognized that by creating a UWB waveform by mixing two derivative data streams running at one-half a chipping rate of the original data stream, that power consumption can be reduced within the UWB device.
These and other objects are achieved according to the present invention by providing a novel circuit for generating wavelets that is highly integratable, and a two-mixer approach for using the wavelets for encoding a data stream while canceling the leakage introduced by non-ideal analog mixers.
In one embodiment, the wavelet generator uses two pulses, an early pulse and a late pulse, from a pulse generation circuit, that when mixed with a positive or a negative voltage in a conventional differential mixer, creates a wavelet that is either positive or negative (i.e., non-inverted or inverted). By mixing the pulse generator output with a stream of data (positive voltage for a xe2x80x981xe2x80x99, negative voltage for a xe2x80x980xe2x80x99), a waveform having a sequence of wavelets is created are transmitted as a UWB signal. In a preferred embodiment, the mixer is a Gilbert cell mixer. In other embodiments, the mixer is, for example, a diode bridge mixer, or any electrically, optically, or mechanically-driven configuration of switching devices including, for example, an FET, a heterojunction, a bulk semiconductor device, or a micro-machine device.
In one embodiment of the two-mixer configuration for creating the UWB waveform, the noise introduced by the analog devices is canceled by whitening the output of the first mixer by mixing it with a white signal at the second mixer. In this embodiment, the data stream is divided into two derivative data streams, one of which is mixed with the pulse generator at the first analog mixer, and the other is mixed with the wavelets created by the first mixer. Since the derivative data streams are sufficiently white, mixing the result of the first mixer with the second derivative data stream will spread the unwanted spikes introduced by the first mixer. Moreover, in this embodiment, the two derivative data streams are at one-half the chipping rate of the original data stream, thereby reducing the power consumption by running at a lower clock rate.
In a second embodiment of the two-mixer configuration, a noisy signal is applied through an exclusive OR (XOR) to a single data stream. The result is then mixed with the pulse generator to create the wavelets at a first mixer. The result of the first mixer is then mixed with the noisey signal at the second mixer, again, to spread the unwanted spikes introduced by the first mixer.
Consistent with the title of this section, the above summary is not intended to be an exhaustive discussion of all the features or embodiments of the present invention. A more complete, although not necessarily exhaustive description of the features and embodiments of the invention is found in the section entitled xe2x80x9cDESCRIPTION OF THE PREFERRED EMBODIMENTSxe2x80x9d as well as the entire document generally.